In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a water removal, system. A typical forming section of a papermaking machine includes an endless traveling papermaking fabric or wire, which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper. Many factors influence the rate at which water is removed which ultimately affects the quality of the paper produced.
It is well known to continuously measure certain properties of the paper material in order to monitor the quality of the finished product. These on-line measurements often include basis weight, moisture content, and sheet caliper, i.e., thickness. The measurements can be used for controlling process variables with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to disturbances in the manufacturing process. The on-line sheet property measurements are often accomplished by scanning sensors that periodically traverse the sheet material from edge to edge.
It is conventional to measure the moisture content of sheet material upon its leaving the main dryer section or at the take up reel employing scanning sensors. Such measurement may be used to adjust the machine operation toward achieving desired parameters. One technique for measuring moisture content is to utilize the absorption spectrum of water in the infrared (IR) region. A monitoring or gauge apparatus for this purpose is commonly employed. Such an apparatus conventionally uses either a fixed gauge or a gauge mounted on a scanning head which is repetitively scanned transversely across the web at the exit from the dryer section and/or upon entry to the take up reel, as required by the individual machines. The gauges typically use a broadband infrared source such as a quartz tungsten halogen lamp and one or more detectors with the wavelength of interest being selected by a narrow-band filter, for example, an interference type filter. The gauges used fall into two main types: the transmissive type in which the source and detector are on opposite sides of the web and, in a scanning gauge, are scanned in synchronism across it, and the scatter type (typically called “reflective” type) in which the source and detector are in a single head on one side of the web, the detector responding to the amount of source radiation scattered from the web. While it is most common to position IR moisture gauges in the more benign dry-end environment, similar gauges are also employed in the wet-end of the papermaking machine. The wet-end moisture gauges are typically located at the end of the press section or the beginning of the dryer section. Gauges in these locations are useful for diagnosis of press and forming sections of the paper machine, or for “setting up” the web for entry into the dryer section.
In operation, the intensity of the infrared beam is not only dependent upon the moisture content and basis weight of the web, the absorption of infrared radiation by the moist web also varies with wavelength. The water and web fibers absorb certain wavelengths of the infrared spectrum more effectively than other wavelengths so that there are absorption peaks and valleys at various wavelengths along the spectrum. Moreover, these peaks and valleys shift to shorter wavelengths with increases in web temperature and to longer wavelengths with decreases in web temperature. It is important for infrared moisture measuring devices to compensate for shifts in the infrared absorption spectrum resulting from changes in web temperature. Because on-line paper web temperatures may range from 10° C. to as high as 100° C., the moisture measurements of these devices are subject to significant error otherwise.
U.S. Pat. No. 4,928,013 to Howarth et al. describes an infrared moisture sensor with two band pass filters which are selected to compensate for web temperature changes. In this sensor, a first band pass filter, associated with a measure detector, is selected so that it is approximately centered about the infrared absorption peak for water, at about 1.93 microns. As the web temperature increases, the intensity of detected infrared radiation increases at the long wavelength side of the pass band filter, while an approximately equal decrease in the detected infrared occurs at the opposite short wavelength side of the pass band. With this technique, the total amount of infrared radiation reaching the measure detector is said to be substantially insensitive to web temperature. A second band pass filter, associated with a reference detector, is selected so that it is in a region of the infrared spectrum close to the measure filter but far enough away from the measure filter that it is not effected by water absorption. Signal variations on this reference channel will be dominated by web variation other than those associated with water, these same losses are also present in the measure channel. These non-water dependent signal losses are likely dominated by scattering which will be dependent upon the basis weight of the sheet.
U.S. Pat. No. 5,124,552 to Anderson describes a technique for determining the moisture content of a web by detecting the amount of infrared radiation transmitted through the web, or reflected from the web, in three separate wavelength regions of the infrared spectrum. Temperature insensitivity is said to be achieved by carefully selecting the temperature response of a first band pass filter, the measurement filter, and a second band pass filter, the reference filter, based on the maximum basis weight and maximum moisture content of the web and further compensating for any remaining temperature sensitivity with a third band pass filter, the temperature correction filter.
In a papermaking, the moisture sensor output is typically used to control the moisture level via an actuation mechanism, e.g., injection of steam into the paper, which in turn influences the sheet temperature. Moreover, with prior art techniques of measuring moisture, minimizing temperature sensitivity comes at the expense of moisture sensitivity. The art is in search of an improved moisture temperature sensor. It is desirable to eliminate this moisture/temperature cross sensitivity.